In a compressed air energy storage (CAES) system for producing electricity, the gas turbine type power plants employ large, electrically-driven compressors to compress the air before mixing it with the gas and burning in the combustor to turn the turbine and generate electricity. A large percentage of the power produced by the turbine, e.g. 30 to 35%, is used to compress the air. Since the power demand varies throughout the day, excess power is available from the power plant during off-peak hours. To avoid peak and valley hours and operate the power plant more efficiently, it can be run continually at a relatively constant output but the surplus electrical energy produced during off-peak periods is stored in the form of compressed air. Preferably, the compressed air is stored in a subterranean cavern at a constant pressure. Then, when the demand for electricity is high, compressed air is drawn from the cavern for use with gas in the combustor. The cavern is maintained at a relatively constant pressure by hydraulically coupling it to a compensating reservoir at ground level via a subterranean connecting tunnel and a generally vertical hydraulic compensating shaft.
Air goes into solution with or is absorbed by water at a rate which increases with the air pressure up to a pressure of about 1000 psi. Thus, in a hydraulically compensated compressed air energy storage system, water in the cavern absorbs air at the cavern pressure. The dissolved air in the cavern water diffuses through the water in the connecting tunnel and into the compensating shaft. Pumping air into the cavern and withdrawing it from the cavern causes water to be moved to and from the cavern through the hydraulic compensating system. In the vertical shaft, the dissolved air in the water goes out of solution and forms bubbles, which grow in size and become more numerous as they rise through the shaft toward the surface reservoir.
Uncontrolled, the air bubbles force the water in the upper region of the shaft out of the shaft, a phenomenon often referred to as the "champagne effect." Since the vertical compensating shafts are quite large, for example 2000 feet deep and 12 feet in diameter, the forces and spray produced by the champagne effect are substantial. If uncontrolled, these forces can undesirably stress the upper portion of the compensating shaft and, together with the spray, can adversely affect the reservoir and low land surrounding the shaft.
The champagne effect also causes the effective hydrostatic head of the water column in the shaft to fluctuate, which in turn causes fluctuations in the air pressure within the cavern. Gas turbines, which are used with CAES power plants, need a continuous supply of compressed air at a constant pressure to insure operational stability. Fluctuating air pressures are, therefore, detrimental to the operation of the CAES system power plant and should be minimized.
Several solutions have been suggested for reducing or eliminating the champagne effect within the hydraulic compensating shaft. One method is to prevent absorption of air into the water in the cavern. This may be accomplished by providing an oil film over the water or a solid barrier such as balls or pads floating on the water surface. These solutions have several drawbacks. The liquid film may escape through or be absorbed by the cavern walls and vapors of the liquid are a potential fire or safety hazard. Solid barriers may be attacked by bacteria and microorganisms in the cavern or they may accumulate at one end of the cavern and block the connecting tunnel.
Another method for controlling pressure fluctuations caused by the champagne effect is to maintain a continuous hydrostatic head of air-free water on the water in the cavern. By segregating the bubbles into one path which extends over the depth of the hydraulic compensating shaft, a continuous column of air-free water is formed in the entire compensating shaft so that the air pressure in the cavern remains relatively constant. It has been proposed to effect such a segregation of the air bubbles with arrays of louvers or plates placed across the entire cross section of the shaft at axially spaced points to guide the bubbles along one side of the shaft. Such a system blocks access to the shaft, making maintenance difficult, and may be damaged by water hammer shocks.
In another approach, helical baffles, or vanes, are mounted along the axis of the shaft. They direct the heavier, bubble free water towards the wall of the shaft while the air bubbles flow into a series of coaxial tubular stacks at the center of the shaft and rise therein towards the surface. However, this approach also obstructs access to the shaft and requires an extensive support system for the centrally mounted helical baffles and tubular stacks.
What is needed, therefore, is an environmentally sound, cost effective system for substantially eliminating water spouts at the surface of the compensating shaft and for eliminating pressure fluctuations within the cavern of a hydraulic compensating system caused by the champagne effect in the upper portion of the hydraulic compensating shaft.